There is a major requirement for thermoplastically processable elastomers which combine the processing properties of thermoplastics with the elastic properties of the irreversible crosslinked materials (often also termed thermoset materials), e.g. in the form of conventionally crosslinked rubber products.
The person skilled in the art is aware of various classes of what are known as thermoplastic elastomers.
One class of these thermoplastic elastomers is provided by what are known as “TPEs”. These are thermoplastic elastomers based on polymers which simultaneously have a) a crystalline and/or amorphous phase whose melting point or glass transition temperature is above room temperature, and also b) have an amorphous phase whose glass transition temperature is below room temperature, the crosslinking of the phases a) and b) taking place by way of the thermoplastic phase a), physical crosslinking being involved here.
Another class of the thermoplastic elastomers is provided by what are known as “TPVs”. These are thermoplastic vulcanizates which comprise mixtures composed of a) crystalline and/or amorphous polymers whose melting point or glass transition temperature is above room temperature and b) amorphous polymers whose glass transition temperature is below room temperature, the amorphous polymers b) having been chemically crosslinked, and this mixture being present with co-continuous phase morphology or having the solid phase as continuous phase.
From the point of view of the user, there is a major requirement for products which combine high-temperature resistance with oil resistance and barrier properties. The conventional products hitherto mainly involve thermoplastic vulcanizates based on polyamides or on polyesters or polypropylene as thermoplastic phase. In these TPVs, there is chemical crosslinking of the elastomeric phase, for example via resins, peroxides, sulphur, diamines or epoxides. In these systems, precondition has to be met in order to obtain the desired property of thermoplastic processability, is that of a continuous or at least co-continuous phase of the thermoplastic material, where the thermoplastic must substantially surround the elastomer phase. In order to achieve this, the elastomeric phase is irreversibly chemically crosslinked during the preparation process. For the desired products intended for use at very high temperatures, a necessary precondition is the use of high-melting-point thermoplastic phases whose melting point or glass transition temperature is >200° C., and this considerably restricts the choice of crosslinking systems and preparation processes. A multistage process has therefore frequently been needed hitherto to achieve the desired properties and the necessary phase morphologies. Reproducibility of the results is accordingly poor.
The crosslinking system used exerts considerable influence.
A known main class of conventional crosslinking systems is provided by free-radical crosslinking systems which operate with use of organic peroxides, with additional use of co-agents to improve free-radical yield.
Another class of even more widely used crosslinking systems is provided by the sulphur crosslinking systems. As is well known to the person skilled in the art, these can be used in a large number of different compositions.
Less commonly used crosslinking systems are those based on amino crosslinking by means of sterically hindered amines, e.g. [Diak 1] (hexamethylenediamine carbamate) in combination with [DOTG] (diorthotolylguanidine). These crosslinking systems are recommended specifically for the crosslinking of elastomers containing carboxy groups, e.g. AEM (e.g. in the form of VAMAC®).
All of the abovementioned crosslinking systems bring about irreversible crosslinking of the elastomeric phase.
A substantial disadvantage of crosslinking by means of peroxides for the possibility desired above of crosslinking above 200° C. is that the crosslinking must take place with exclusion of air in order to avoid a reduction in crosslinking efficiency via reaction with atmospheric oxygen, the discernible result of which is, for example, formation of surface tack. Another significant aspect of peroxide crosslinking is the firm relationship between temperature and decomposition rate. While achievement of a suitable crosslinking rate has a relatively low temperature (<180° C.) is relatively easy to achieve via selection of a suitable peroxide, it is very difficult to find suitable peroxides, i.e. peroxides that react selectively with the rubber phase, for controlled crosslinking reactions at or above 200° C. Peroxides commonly used in the rubber industry for the crosslinking of rubbers are unsuitable for these high-temperature reactions by virtue of explosive decomposition kinetics. This method cannot ensure homogeneous dispersion of the peroxides in the elastomeric phase during the mixing process of thermoplastic and elastomer phase at high temperatures. Other high-temperature peroxides often exhibit only unsatisfactory crosslinking efficiency with the rubbers under consideration, or are not commercially available. Products produced by means of peroxidic crosslinking often moreover feature strong undesirable odor, deriving from the decomposition products of the peroxides.
Another method of carrying out high-temperature crosslinking reactions uses chemical condensation or chemical addition processes on reactive polymers which by way of example have hydroxy or carboxy groups as functional groups, by means of reactive chemical cross-linking agents based on di-, tri- or polyfunctional epoxides, amines, carboxylic salts, or isocyanates. A disadvantage here is that the products used often have considerable toxicity.
The constituents needed for typical sulphuric crosslinking are many and varied and encompass sulphur, sulphur donors, accelerators, retarders, antireversion agents and other substances. However, vulcanization temperatures beyond 180° C. are not commonly used because control of the reaction and of the process is very difficult at temperatures as high as this.
Production of thermoplastic vulcanizates based on PP and EPDM by means of what is known as resin crosslinking often uses phenyl-formaldehyde resins together with stanus dichloride (SnCl2) as Lewis acid catalyst. Although these systems are very widespread they have the serious disadvantage of producing severely discoloured products often with a yellowish brown hue and of liberating corrosive chlorine compounds. They therefore have limited applicability.
The term reactive crosslinking is used for crosslinking processes during high-temperature mixing of elastomers or of elastomers and thermoplastics. This step is significant for the production of thermoplastic vulcanizates.
Another new class of thermoplastic elastomers is provided by ionically crosslinked thermoplastic elastomers, which are also termed thermoplastic ionomers. These involve mixtures composed of ionomeric thermoplastics and of elastomers containing carboxy groups or containing sulphonic acid groups and mostly based on polypropylene-(PP)- and polyethylene-(PE)-based copolymers whose phases are coupled by way of ionic bonds. These ionically linked thermoplastic elastomers have been previously disclosed.
By way of example, WO-A-03/020820 describes blends composed of a polyamide with a hydrogenated carboxylated nitrile rubber based on a nitrile monomer, on a diene comonomer, and also on an unsaturated carboxylic acid as termonomer.
The literature has previously described the use, for the crosslinking of elastomers bearing carboxy groups, of mixtures composed of known crosslinking systems, e.g. those based on sulphur compounds, which bring about covalent crosslinking, and metal salts, which bring about additional ionic crosslinking.
U.S. Pat. No. 4,508,867 discloses vulcanizable rubbery compositions which encompass a crystalline polyamide, and also a synthetic, rubbery polymer based on acrylnitrile or methacrylnitrile, on butadiene and on one or more α,β-unsaturated carboxylic acids as monomers and moreover also comprise a combination of active sulphur-vulcanization agents, and also of non-polymeric additives based on metal salts/compounds. The amounts of these non-polymeric additives are from 0.1 to 15% by weight, based on the polyamide, of an additive which has been selected from the halides of lithium, magnesium, calcium and zinc, and about 1-10 parts by weight, based on 100 parts by weight of all of the polymeric materials, of an additive which has been selected from the oxides and hydroxides of magnesium, calcium, barium and zinc and from the peroxides of calcium and zinc. U.S. Pat. No. 4,508,867 states that these additives affect the melting point of the polyamide or else the compatibility of the polyamide and of the rubbery polymer.
U.S. Pat. No. 6,566,463 discloses specific metal salts of unsaturated carboxylic acids which together with peroxides are suitable for the crosslinking of rubbers. The specific metal salts are obtained via reaction of 2 mol of a monobasic unsaturated carboxylic acid and 2 mol of a dibasic unsaturated carboxylic acid with 3 mol of a divalent metal oxide.
The sole use of divalent metal salts for achievement of ionic crosslinking is also known from the literature:
Polymer Engineering and Science, May 1999, Vol. 39, No 5, 963-973 discloses ionomeric thermoplastic elastomers based on ionomeric polymer blends of zinc salts of maleated polypropylene (“Zn-mPP”) and of maleated EPDM rubber (“Zn-mEPDM”). The ionic crosslinking at the interfaces is brought about via addition of zinc oxide and stearic acid.
Journal of Applied Polymer Science, Vol. 86, 2887-2897 (2002) discloses that a polypropylene-based elastomer can be grafted with maleic anhydride, and the resultant maleated product can be crosslinked via additional of metal salts, such as aluminium stearate, magnesium stearate, calcium stearate, zinc stearate, potassium stearate, sodium stearate, magnesium hydroxide, zinc oxide or zinc sulphide.
Polymer 41 (2000) 787-793 moreover says that ionomeric polyblends composed of zinc-oxide-neutralized carboxylated nitrile rubber (“Zn-XNBR”) and of zinc-oxide-neutralized poly(ethylene-co-acrylic acid) (“Zn-PEA”) behave as ionomeric thermoplastic elastomers. Here again, the ionic crosslinking at the interfaces is brought about via addition of zinc oxide and stearic acid.
J. Applied Polymer Science, Vol 87, 805-813 (2003) also says that carboxylated nitrile rubber can be crosslinked ionically using calcium oxide and stearic acid. According to Polym. Int. 49, 1653-1657 (2000) carboxylated nitrile rubber is converted into an ionically crosslinked elastomer using calcium oxide, magnesium oxide or zinc oxide in the presence of dioctyl phthalate or dimethyl sulphoxide as plasticizer.
However, the result of this crosslinking of the elastomer phase exclusively via oxides of divalent metals and their organic salts is polymer blends which feature high loss factors in dynamically loaded products and with this an undesired high level of dynamic heating. Although synergies are found for the crosslinking reaction with zinc oxide and with organic acids in the crosslinking of blends of various thermoplastic and elastic ionomers, partial dissociation of the ionic carbons in the temperature range as low as from 170° C. to 180° C. is presumed and is inimical to use in ionomers having high-temperature resistance.
Journal of Elastomers and Plastics, Vol. 33, 196-210 discloses that chlorosulphonated polyethylene (CSM) can be reacted with aluminium oxide in the presence of stearic acid, giving an ionic elastomer. There is also a description of a “mixed crosslinking reaction”, i.e. formation of a vulcanizate having two different types of crosslinking, using dicumyl peroxide (DCP) and also using aluminium oxide/stearic acid. Mixed crosslinking using a combination of DCP and aluminium oxide/stearic acid is also required for the production of blends: an appropriate blend is obtained via intensive mixing of a masterbatch composed of CSM/aluminium oxide/stearic acid and of a masterbatch composed of ethylene-vinyl acetate copolymer (EVA)/DCP. However, when the purely ionic crosslinking reaction is carried out using aluminium oxide/stearic acid, it is said that materials with inadequate compressive deformation properties are obtained.